The Way The World Searches for Extraterrestrial Life May Be Holding Back Discoveries

One of science’s greatest false alarms occurred on Aug. 7, 1996, when NASA announced that scientists had discovered “strong circumstantial evidence that life once existed on Mars.” That evidence was a scattering of tiny tube-like structures that looked for all the world like fossilized bacteria inside a small meteorite from Mars. Television networks interrupted regular broadcasts to cover the news, and President Clinton convened a Rose Garden press conference to comment on it.

The rock, Clinton said, “speaks to us across … billions of years and millions of miles. It speaks of the possibility of life. If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered.”

Alas, it was not confirmed. Further experiments showed that the same tiny formations can be the result of geologic—completely abiotic—processes. With that, exobiologists avoided one of the things they fear the most: the false positive—seeing the existence of life where none exists. But what about the false negative—failing to see, or even rejecting plain evidence that indicates something living might be in the air, the soil, the matrix of a rock? 

That’s the question raised by an intriguing new paper in the journal Nature Astronomy, which argues that our life-detection methods are sometimes flawed, our biases are sometimes roadblocks, and we too often fail to turn over a rock—both literally and metaphorically—to see if something’s living underneath it. That could mean that data gathered suggesting life during multi-billion missions sent specifically to hunt for it—in the deserts of Mars, in the oceans of Saturn’s moon Enceladus and Jupiter’s Europa, on the surface of distant exoplanets—could be overlooked or dismissed before getting a fair and robust scientific vetting. We’re visiting other worlds to look for extant life but setting ourselves up for failure when we get there.

“We should be aware of these false negative results,” said Inge Loes ten Kate, professor of astrobiology and planetary science at Utrecht University and the University of Amsterdam and the lead author of the paper, in a statement that accompanied its release. “It means there are shortcomings in recognizing the existence of life. These shortcomings are not high on the research agenda.”

Read more: Is Colonizing Space the Next Stage of Human Evolution?

One of the biggest causes of false negatives in the search for life, the authors argue, is that the consequences of being wrong do not carry existential risks. In epidemiology, a false negative may allow a lurking virus to range free and take lives; in environmental science it can lead a coastal community to remain unprepared for an approaching typhoon. In exobiology, it leads only to the status quo—living in a world in which life has not yet been discovered anywhere else in the universe.

“Because false negatives in biology do not present…acute risks,” the authors write, “they receive much less attention. Nevertheless, they are still missed opportunities to detect life.”

One of the most-cited examples of the power of no in exobiological experiments occurred during the missions of the Viking 1 and 2 spacecraft, which landed on Mars on July 20, 1976 and Sept. 3, 1976 respectively. During the course of the missions, the twin spacecraft scooped up samples of Martian soil and treated them with nutrients, water, and heat, to see if the samples would show signs of biology, such as releasing radioactively tagged carbon gasses, or absorbing carbon monoxide and carbon dioxide in the soil. In at least two of the studies such possible signs of Martian metabolism were detected, but again, scientists dismissed the results since abiotic processes could account for the reactions too.

“There has never been a mission that looked further into that,” said Ten Kate in a conversation with TIME. “I would love to see a mission going into that direction again.”

It’s also possible that our detection methods are imperfect, with gaseous byproducts of biology getting hidden or obscured by background atmospheric gasses. Using a gas chromatograph mass spectrometer, for example, the spectral biosignatures of carbon dioxide and methane may overlap, leading to the gasses being confused with each other and signs of life being dismissed or lost.

“We really [have to] look at an environment from all different viewpoints,” says Ten Kate. “We should be conducting our studies differently from the way we’re conducting them now, so as to increase the odds that we don’t overlook biosignatures.”

We are also hobbling ourselves by limiting our ambitions. Last September, a paper in the journal Nature reported that the Perseverance Mars rover had discovered a curious rock in a dry delta that was once part of a river. The rock was streaked in a range of colors—red, green, purple, and blue—flecked with poppy-seed-like dots and decorated with what the Perseverance scientists compared to dull yellow leopard spots. As TIME wrote at the time, the rover’s instruments confirmed that the red is iron-rich mud, the purple is iron and phosphorous, the yellow and green are iron and sulfur—all of which are food for hungry microbes. On Earth, those colors along with the poppy seeds and leopard spots are often left behind by microbial metabolism.

“This finding by Perseverance … is the closest we have ever come to discovering life on Mars,” said then-acting NASA Administrator Sean Duffy. “The identification of a potential biosignature on the Red Planet is a groundbreaking discovery.”

But it is destined—at least for now—to remain a partial discovery. To determine if the rock hosts or hosted Martian life, it has to be examined by an on-the-site astronaut or brought back to Earth for hands-on study. Astronauts on Mars are many years if not decades away. As for a mission to return Martian samples, that was—but is no longer—in its early stages. Ever since Perseverance landed on Mars in early 2021, it has been gathering up samples of Martian soil and rock, caching them in small titanium tubes, and leaving them scattered across the Martian surface like Easter eggs for a future robotic mission to collect and bring home. But funding for the Mars Sample Return mission was zeroed out of the fiscal 2026 budget, leaving the tubes in place with nobody to collect them.

“We need further research, and of course we would need sample return,” says Ten Kate. “I don’t think those are missed opportunities yet, because everyone is interested. Understanding those types of environments will hopefully prevent that from being a false negative.”

Then, of course, we have to be careful with how we handle possible extant life—lest it become former life. If the Martian soil the Vikings treated with gas and nutrients contained microbes, we might have killed them in the process of looking for them.

“If there were bacteria, the bacteria might have drowned because the growth medium contained too much water,” says Ten Kate. “On Earth we also have microbes that inhale their water directly from the atmosphere, so if you submerge them in growth medium, they die too.”

Exobiologists do have reason to be cautious about ringing the life-in-space bell prematurely. The screaming headlines followed by the official oops that came in the wake of the announcement of the 1996 Mars rock is something scientists are justified in wanting to keep off their resumes. But research is about taking chances too—being willing to say yes or at least maybe to the possibility of life if the weight of evidence seems to be there. 

“Space missions and instruments are designed to detect potential signs of life,” said Ten Kate in the official statement, “but the risk of overlooking something is not taken into account. The search for signs of life should go hand in hand with better-defined questions and testable hypotheses.”